A terminal here means any device which is intended to be electrically connected to a subscriber line of the telephone network and to be used to communicate information through the telephone network. E.g. a telephone, a modem, a telefax termninal, an answering machine.
A subscriber line of the telephone network here means that part (the telephone line) of the telephone network, which a telephone company or other corporation maintaining the telephone network supplies to a customer for the terminal. Physically the subscriber line normally comprises one conductor pair. To the customer the subscriber line usually "appears" as a telephone socket, to which he connects the terminal in question.
An exchange here means that part of the telephone network, which in a convenient way performs the switching between different subscriber lines in order to communicate information. The exchange can be an exchange covering e.g. a city or another large geographical area, or an internal exchange of a company or a building.
When a terminal connected to the subscriber line is activated, it will first supply the required information (the telephone number), with the aid of which the exchange connects the terminal in question to another terminal, and when the terminal terminates its activity (changes into a non-active state), the exchange disconnects the connection, i.e. it separates the terminals from each other.
The electrical characteristics of the subscriber line are such that normally a plurality of terminals can be simultaneously connected to the same subscriber line. This is due to the fact that the terminal receives current (typically 10 to 100 mA) from the subscriber line only when it is operating (active). With the aid of this current the exchange recognizes that the terminal in question is in use. When the terminal ends it activity it will cut off the line current, whereby the exchange recognizes that the line is not used anymore, and then disconnects the connection.
Of those terminals connected to a single subscriber line only one can be in operation (active) at a time. If two or more terminals are simultaneously active they will generally interfere with each other, or totally prevent appropriate operation. Different switching arrangements have been developed in order to eliminate this problem, but even they have their disadvantages. Below we discuss some known methods and problems related to their use.
A common solution is to connect the terminals in parallel to the subscriber line, without any special arrangements, and the user is responsible so that only one terminal is used at a time. This is the most common situation, at least in home applications.
One disadvantage is that the user control is not always sufficient, particularly if there are several users, or if there is a great distance between the terminals, or if some of the terminals (e.g. a modem) operate automatically.
In another common solution the terminals are connected to the subscriber line via a relay switch, which operates as follows: when there are no terminals in use all relays are closed, i.e. all terminals are connected to the subscriber line and any one of them can be used. When a terminal is activated and occupies the subscriber line in order to use it, then the relays of the other terminals will open, and then these devices can not operate as long as the first mentioned terminal is active. The relay switch is a separate device, which is directly controlled by the currents conducted by the telephone line and the terminals connected to the line. The terminals will not require any special design.
A disadvantage in the use of the relay switch is that it is not possible to observe the priorities of different terminals, if it for instance would be required that it is always possible to make a call with a telephone connected to the subscriber line, even if it would mean to interrupt the ongoing operation of another terminal during the call.
One solution is to provide the subscriber line with a multiple-way switch, through which only one terminal at a time is connected to the subscriber line. With the switch the user can select a desired terminal at any time; at the same time the use of the other terminals is prevented or interrupted.
A disadvantage is that when the user uses different terminals he must always remember to change the position of the multiple-way switch. This may constitute a substantial inconvenience if there is a great distance between the terminals and between the terminals and the multiple-way switch.
Such solutions are also known in which the terminals form a chain, and each terminal has a connection both for the telephone line and for another terminal. In the prior art solution shown in FIG. 1 the first terminal 1 is connected to the telephone socket (the subscriber line 3) in the usual way, and the second terminal 1' is connected to the connection 8 provided for this in the first terminal, and so on. Then each terminal 1, 1' contains an internal two-way switch or relay 5, 5', which, when the terminal in question is activated, disconnects from the subscriber line those terminals, which are farther behind and connected to the terminal in question. For this purpose the device contains detection and control means 4, 4' which control the switch 5, 5'. The order in the chain will simultaneously determine the priority of the devices: the first device 1 is connected directly to the subscriber line 3 and is always available, and when required it can interrupt the operation of the other devices 1'.
A prior art arrangement shown in FIG. 2 is also known, which is able to partly change this order of importance. Then the device 1 will check before it is activated whether any "next" device connected to this terminal in question is operating, and if so, the terminal in question will wait until the operating device terminate its operation. This check is performed by measuring with the current measurement means 9 the current of the subscriber line 8 conveyed to the next terminal through the terminal 1 in question, and by controlling the detection and control means 4.
On the other hand, if the terminal 1 was already active, the "next" terminal is not able to interrupt its operation, but has to wait.
The disadvantage is that it is possible to connect to the chain only one such terminal which is not equipped with the auxiliary equipment required by the chaining. This device must be connected as the last one in the chain, whereby it is also the last one in the "order of importance". In practice this device is a common telephone, which regarding its use generally is desired to be the first in the order of importance, i.e. a device which always can be used, independently of the other devices.
Further, all known arrangements described above have one common disadvantage: if any terminal tries to activate itself when another one is active, and the latter then ends its operation and connects the first terminal to the telephone line, then the original connection will not be disconnected, because the line current continues to flow via the first terminal, and the exchange is not able to recognize that the connection should be disconnected before a new connection can be made. This can cause substantial inconvenience particularly regarding automatically operating terminals having a design which could not be prepared for such incorrect function.